The invention relates to a system for limiting peak loads in electrical systems and for saving on electrical consumption. More particularly does it relate to an electrical load limiting and load cycling system in which the power usage is continuously monitored and compared to a preset limit. In response to the results of this comparison the device will either add the connected loads until the maximum permissible power usage rate is reached or until the maximum permissible number of loads are added. The device may take no action and remain with the same number of "on" loads or it will drop loads until the power usage rate has been lowered to below the maximum permissible rate or until the maximum permissible number of loads have been dropped.
As is well known by customers of electrical utilities, the customer is often charged by the electric company according to the maximum demand for electrical energy over a billing period. A maximum demand meter is installed on the customer's premises for recording the maximum electrical energy consumed during that time interval. Throughout days and seasons peak loads vary. Some of the loads causing these peaks are known to the customers while others cannot be predicted. Despite the large variation in the demand it is advantageous to the user to maintain the peak load below a predetermined limit. Such a condition for the customer has a billing advantage while the electrical utility is enabled to provide a more adequate service to all its customers.
A number of systems have been devised to deal with the limiting of peak load demands. Some, in order to limit peak demand, sequentially switch loads. One of the obvious requirements of such a system is that the loads to be shed, particularly lower priority loads, are of little importance or value when one considers the benefits of the service rendered by such loads. In fact, it is generally recognized that very few loads fall into this category.
Systems have also been devised to turn loads off in response to the anticipated power usage in synchronization with the power utility demand. Such a system has a tendancy to shed a maximum of loads towards the end of the demand period and no loads at the beginning of the demand period. Not only is this relatively rapid change of loads frequently annoying to the consumer, but it is also objectionable to the power utility as it causes periodic power surges on their lines.
Other systems cycle loads off in response to the actual power being used, but no provision is made to limit the maximum number of stages or loads off. Thus, in such systems and at periods of peak demand, all loads will be shut off. Additionally, there is no provision to limit the minimum number of stages or loads off as a result of which the consumer does not take advantage of the potential savings inherent in maintaining a portion of their loads off at all times.
Among the prior art references to be considered are U.S. Pat. Nos. 3,489,913; 3,566,078; 3,714,453; and 3,769,520. None of the four references cited is particularly pertinent to this invention.